IRAF ZPN patch

Information on the fix for ZPN in IRAF.

Overview

IRAF V2.13 already includes the ZPN driver, earlier versions require the following patch.

IRAF V2.12 and before do not support the ZPN projection. This means that all the operations that use the WCS (e.g. stacking, co-ordinate conversions, etc.) will produce the wrong result. The ZPN projection has been part of the FITS standard (see Calabretta, M.R. & Greisen, E.W. 2002 A&A, 395, 1077) for several years now and it is actually supported by many astronomical software programs.

The quick alternative

Although we strongly recommend fixing the ZPN problem by using the latest IRAF distribution, there is an alternative.

IRAF understands another projection called ZPX which is almost equivalent to ZPN but it is not standard. One possibility is therefore to convert the WCS in the FITS headers from ZPN to ZPX. To do that you can use this IRAF script. One of the main drawbacks of this approach is that the WCS will be interpreted correctly only as long as you are working within IRAF, e.g. DS9 will not understand it WCS anymore.

Note that if you work around the problem by converting to the TAN projection, you get serious astrometric distortion of up 10 arcsec around the outer corners of the detectors.